Mossbauer spectrometer radiation detector

ABSTRACT

The invention is a versatile Mossbauer spectrometer with high efficiencies in both transmission and backscattering techniques. The heart of the invention is a detector which includes an NaI(T1) crystal for detecting radiation caused by the Mossbauer effect. The crystal has a hole in its center to allow the collimated incident radiation to pass through and it is coupled through two quartz optical windows to two photomultipliers such that all of the scintillation radiation produced by the radiation detected by the crystal will flow into the two photomultipliers. When used in a transmission technique the sample or scatterer is placed between the incident radiation source and the detector; and when used in a backscattering technique the detector is placed between the incident radiation source and the sample or scatterer such that the incident radiation will pass through the hole in the crystal and strike the sample.

United States Patent 11 1 1111 3,781,562 Singh Dec. 25, 1973 [54] MOSSBAUER SPECTROMETER 3,612,875 11/1971 Ord 250/360 RADIATION DETECTOR [58] Field Inventor:

Filed:

Jag J. Singh, Yorktown, Va.

Assignee: The United States of America as represented by the Administrator of the National Aeronautics and Space Administration, Washington, DC.

Dec. 8, 1972 1111. c1. GOlj 39/18 of Search 250/276, 361, 367, 250/458, 485, 486

References Cited UNITED STATES PATENTS 10/1958 Wouters 250/367 11/1959 Brannon et al. 250/486 1/1954 Eversole et a1. 250/367 10/1968 Jerko 250/359 6/1966 Cook 250/272 Primary Examiner-Harold A. Dixon Att0rney--Howard J. Osborn et a1.

[ ABSTRACT The invention is a versatile Mijssbauer spectrometer with high efficiencies in both transmission and backscattering techniques. The heart of the invention is a detector; and when used in a backscattering technique the detector is placed between the incident radiation source and the sample or scatterer such that the incident radiation will pass through the hole in the crystal and strike the sample.

6 Claims, 2 Drawing Figures ELECTRO. MECHANICAL SOURCE TRANSDUCER (CONTROL UNIT) 14 ,1 t PHOTOMULTIPLIER l PHOTOMULTIPLIER u 11 2s 2s 24 I v 2s v PRE-AMPLIFIER /29 33 3o "1 [J SINGLE ATTENUATOR INVERTER CHANNEL 4 AMPLIFIER ANALYZER 32 km MOSSBAU ER SP-ECTROMET E R Pmmm m 3.781.562

FIG.1

ELECTRO MECHANICAL SOURCE TRANSDUCER 2 I22 (CONTROL UNIT) P r -23 b 1 I5 13 Pb 4 l6 lz PHOTOMULTIPLIER PHOTOMULTIPLIER 25 25 I 24 v 28 v PRE-AMPLIFIER 1 29 33 L 30 W SINGLE ATTENUATOR INVERTER CHANNEL 4 AMPLIFIER ANALYZER 32 BI MOSSBAUER SPECTROMETER 'MOSS BAUER SPECTROMETER RADIATIO DETECTOR ORIGIN OF THE INVENTION The invention described herein was made by an employeeof the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION this limitation by using a scattering technique in which 1 no special sample preparation is required. At present this technique is not used since the results are not very satisfactory, certainly not as good as in a transmission geometry. There are several reasons for it: (1) low efficiency due to a very low solid angle subtended by previous detector configurations; (2) relatively pronounced interference (in the case of CO by the 122 keV gamma ray-induced fluorescence as a result of low Mbssbauer counting rates and (3) imprecise sourcevelocity definition in previous counting arrangements.

In a transmission geometry, the incident radiation passes through the Miissbauer absorber before falling on the detector at resonance, the absorption cross section rises sharply leading to a considerable reduction in the intensity of the transmitted radiation. The reemitted radiation, because of its-isotropicity, does not en- It is therefore the primary object of this invention to provide a Mtissbauer spectrometer that utilizes a scattering technique which provides better results than previous scattering and transmission techniques.-

Another object of this invention is to provide a radiation counter for use in a Mossbauer spectrometer that has the advantages of subtending a very large solid angle at the scatterer and of seeing only the scattered radiation.

Other objects and advantages of this invention will become apparent hereinafter in the specification and in the claims.

SUMMARY OF THE INVENTION The invention is a versatile Mi'issbauer spectrometer that is efficient both in transmission and reflection ge-' ometries. Itincludes a radiation detector consisting of a 0.6 inch by 2 inches by 2 inches NaI(T1) crystal with a 0.6 diameter hole in its center. The crystal encapsulation incorporates 1.5 inch diameter, 0.005 inch thick front and back beryllium windows that permit .easy penetration by theconversion X-rays and the reemitted 14.4 keV gamma rays into the sensitive region of the ter.

tirely compensate for loss, in the forward direction, due

to absorption out ofthe incident beam. The transmitted radiation spectrum, therefore, shows a net reduction in the counting rate at resonance velocity. Since only a certain fraction, Mdssbauer fraction, of the incident beam exhibits resonance fluorescence, one is necessarily faced with a large nonresonant component at the detector. This nonresonantcomponent has the net effect of reducing the SIN ratio, and consequently the resolution, in the transmission spectra. The situation could be considerably improved if the detector did not see nonresonant component of the incident radiation. One way of achieving this is to use the scattering geometry in which the detector sees only the resonantly scattered radiation. Scattering technique in transmission geometry has not been used because the absorber must be thin, just as in the conventional absorption technique,

- to let the incident, as well as the reemitted, radiation ploited because of the extremely long counting periods needed for good statistical accuracy in the spectra.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cutaway view of the detector portion of this invention; and.

FIG. 2 is a block diagram showing the use of the de tector in FIG. 1 in a backscatteringlgeometry.

DETAILED DESCRIPTION OF THE, INVENTION 7 Turning now to the embodiment of the invention selected for illustration in the drawings the number 11 in FIG. 1 designates an NaI(T1) crystal. The dimensions of crystal 11 are Zinches by 2 inches by 0.6 inch with a 0.6-inch diameter hole 12 at its center. Crystal 11 is coupled through one-fourth inch thick quartz optical windows 13 and 14 to photomultipliers 15 and 16, re-

spectively. The crystal 11 is completely encapsulated signals and provision for adjusting the focusing voltage to equalize the multiplier gains.

Even though crystal 11 has been disclosed as having a rectangular shape, it can be many different shapes. The only' requirement is that crystal 1 1 be shaped on its two ends to provide for direct coupling of optical windows (light guides) to transmit all the scintillation radiation to the windows of the two photomultipliers. Also crystal 11 can have dimensions different from those disclosed without departing from the invention.

The detector disclosed in FIG.- 1- can be used in both transmission and reflection geometries. P16. 2 is a block diagram of the use of the detectors in FIG. 1 in a reflection or backscattering geometry. In this use, an electro-mechanical source transducer 21 controlled by control units (not shown) produces motion of the 'CO" source 21. The radiation emitted by this source is collimated by a collimator assembly 23. The collimated radiation beam 22 from the source 21 passes through the hole 12 of the detector and strikes the specimen 24 being studied. Specimen 24 is shown stationary; however, it could have motion as well as source transducer 21. The resulting backscattering radiation 25 is detected by the Nal(T1) crystal and then passed through the quartz optical windows'to photomultipliers 15 and improved sensitivity of the Mijssbauer spectrometers.

16 which have a high voltage Hv applied to them. The

solid angle subtended by the detector, at the point of beam 22 incidence on scatterer 24, can be maximized by adjusting the distance between the scatterer and the detector. The outputs'of photomultipliers 15 and 16 are applied to a summing network consistingof resistors 26, 27 and a potentiometer 28. The purpose of potentiometer 28 is to equalize the sensitivities of the two photomultiplier channels. The sum of the outputs from photomultipliers appears at the slider of potentiometer 28. This sum voltage is amplified by preamplifier 29 and amplifier 30 and then applied to a single channel analyzer 31. The SCA 31 limits are adjusted to accept all pulses in the range of from 5 to 15 keV. The output of the SCA is applied, through an inverter 32 and an at-' Also, it can be used under environmental conditions without any special preparation of the sample or scatterer.

It is to be understood that the form of the invention herewith shown and described is to be taken as a preferred embodiment. Various changes can be made without departing from the spirit or scope of the invention as defined in the subjoined claims. For example, the shape, dimensions and composition of crystal 11, the source of incident radiation, and the balancing circuit for the two photomultipliers can be different from that disclosed without departing from the invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

I. A detector for use in a Mdssbauer spectrometer wherein a beam source of gamma rays irradiates a sample for producing secondary gamma rays characteristic of the Mds sbauer effect comprising:

a crystal having a composition suitable for detecting radiation caused by the Mdssbauer effect and having a hole in it for passing the substantially unattenuated primary radiation of said beam source of gamma rays, and two photomultipliers connected to two sides of said crystal such that substantially all of the scintillation radiation produced by the radiation detected by said crystal flows out of said crystal into said two pliers.

for equalizing the sensitivities of the two photomulti- 

1. A detector for use in a Mossbauer spectrometer wherein a beam source of gamma rays irradiates a sample for producing secondary gamma rays characteristic of the Mossbauer effect comprising: a crystal having a composition suitable for detecting radiation caused by the Mossbauer effect and having a hole in it for passing the substantially unattenuated primary radiation of said beam source of gamma rays, and two photomultipliers connected to two sides of said crystal such that substantially all of the scintillation radiation produced by the radiation detected by said crystal flows out of said crystal into said two photomultipliers.
 2. A detector according to claim 1 wherein the composition oF said crystal is NaI(T1).
 3. A detector according to claim 1 wherein said crystal is coupled to said two photomultipliers through two quartz optical windows.
 4. A detector according to claim 3 including a beryllium window on each side of said hole.
 5. A detector according to claim 3 including means for summing the outputs of said two photomultipliers.
 6. A detector according to claim 5 including means for equalizing the sensitivities of the two photomultipliers. 